1. Field of the Invention
This invention relates to a developing device for developing an electrostatic image formed on an image bearing member with a developer, and the developing device is preferably used in an image forming apparatus of the electrophotographic type, the electrostatic recording type or like type.
2. Description of the Related Art
As the electrophotographic method, there are known a number of methods as described in U.S. Pat. No. 2,297,691, Japanese Patent Publication (Koukoku) No. 42-23910 and Japanese Patent Publication (Koukoku) No. 43-24748, and generally, by the utilization of a photoconductive material and by various means, an electrical latent image is formed on the surface of a photosensitive member which is a latent image bearing member, and then the latent image is visualized as a toner image by the use of a toner which is a developer, and as required, the toner image is transferred to a transfer material which is a recording medium such as paper, whereafter the transfer material is subjected to the fixing process by heating or solvent vapor or the like to thereby obtain a copy.
There are also known various methods of visualizing an electrical latent image by the use of a toner, and as developing methods, there are known a number of developing methods such as a magnetic brush developing method, a powder cloud method, a fur brush developing method and a liquid developing method.
In these developing methods, particularly magnetic brush development, cascade development, liquid development, etc. using a developer consisting chiefly of a toner and a carrier are widely put into practical use. Any of these methods is an excellent method which can obtain good images relatively stably, while on the other hand, they have common disadvantages peculiar to a two-component developer such as the deterioration of the carrier and the fluctuation of the mixing ratio of the toner and the carrier.
In order to avoid such disadvantages, various developing methods using a mono-component developer consisting of only a toner (hereinafter referred to as the mono-component developing method) have been proposed. For example, in U.S. Pat. No. 3,909,258, there is proposed a method of developing by the use of a magnetic toner having electrical conductivity. This is to cause a sleeve which is a cylindrical and electrically conductive developer carrying member having magnetism therein to carry a magnetic toner thereon, and bring the magnetic toner into contact with an electrostatic latent image which is a latent image on a latent image bearing member to thereby develop the latent image. At this time, in a developing area which is a portion in which the latent image bearing member and the sleeve are opposed to each other, an electrically conducting path is formed between the surface of the latent image bearing member and the surface of the sleeve by toner particles, and charges are directed from the sleeve to the toner particles via this electrically conducting path, and the toner particles adhere to the electrostatic latent image by the Coulomb""s force between the sleeve and the electrostatic latent image on the latent image bearing member, whereby the electrostatic latent image is developed. This developing method using the magnetic toner having electrical conductivity is an excellent method avoiding the problems peculiar to the conventional two-component developing method, while on the other hand, it has the disadvantage that since the toner is electrically conductive, it is difficult to finally electrostatically transfer the toner image from the latent image bearing member to a recording medium such as plain paper after the electrostatic latent image has been visualized as a toner image.
In order to solve this problem, as a developing method using a high-resistance toner capable of electrostatically transferring a toner image from a latent image bearing member to a recording medium, Japanese Patent Application Laid-Open No. 52-94140 shows a developing method utilizing the dielectric polarization of toner particles. Such a method, however, has the disadvantages that essentially the developing speed is low and the density of the toner image by development is not sufficiently obtained, and has been difficult in practical use. As another method using a high-resistance magnetic toner, there is known a method in which toner particles are triboelectrically charged by the friction between toner particles and the friction between the toner particles and a sleeve, and the toner particles are brought into contact with a latent image bearing member to thereby develop a latent image on the latent image bearing member. These methods, however, have the disadvantage that the frequency of the contact between the toner particles and the sleeve or the like is small and triboelectric charging is liable to become insufficient, or when the Coulomb""s force between the toner particles and the sleeve is strong, the toner particles are liable to cohere on the sleeve, and it is pointed out that these methods have many difficulties in practical use.
In contrast, in Japanese Patent Application Laid-Open No. 54-43036, there is proposed a novel developing method which has eliminated the above-noted disadvantage. This is to apply a toner very thinly onto a sleeve, and triboelectrically charge the toner, and then make the toner very proximate to an electrostatic latent image under the action of a magnetic field and oppose the toner to the electrostatic latent image without bringing the toner into contact with the electrostatic latent image, and develop the electrostatic latent image.
According to the method, it is made possible to adopt a construction of applying a magnetic toner very thinly onto the sleeve to thereby increase the frequency of contact between the magnetic toner and the sleeve, and induce in the toner a triboelectric charge amount necessary to be used for development.
According to our investigation of the imparting of charges to the toner in the mono-component developing process, it has been found that the behavior of the toner in the charge-imparted portion is as follows.
FIG. 4 of the accompanying drawings shows an example of a developing device using the magnetic toner.
Such a developing device, as shown in FIG. 4, is provided with a developing sleeve 101 which is a developer carrying member disposed for rotation in the direction of arrow and using a nonmagnetic member, a permanent magnet 101b fixed to the interior of the developing sleeve 101, a magnetic blade 202 which is developer regulating means using a magnetic member, a developing container 103 for containing a magnetic toner therein, and a conveying member 104.
The magnetic blade 202 is disposed so that the distance thereof to the developing sleeve 101 may be a constant value W. Generally, it is often the case that the distance W is set to a value within the range of 100 xcexcm-1 mm.
In the developing device shown in FIG. 4, the developing sleeve 101 is coated with a thin layer of magnetic toner T, as shown in FIG. 6 of the accompanying drawings. The layer thickness of this toner layer is determined by the position of a cut line L shown in FIG. 6.
According to our investigation, it has been found that when the magnetic toner T passes between the developing sleeve 101 and the magnetic blade 202, charges are imparted to the magnetic toner T. Also, it has been found that the behavior of the magnetic toner T during that time is as follows.
As shown in FIG. 5 of the accompanying drawings, supposing planes perpendicular to a straight line linking the developing sleeve 101 and the magnetic blade 202 together, and defining a plane near to the magnetic blade 202 as S1, and a plane near to the developing sleeve 101 as S2, the magnetic flux density on the plane S1 is greater than the magnetic flux density on the plane S2 because generally the width of the magnetic blade 202 is made narrow as compared with the width of the permanent magnet 101b. Accordingly, the magnetic toner T receives a force in the direction indicated by the arrow in FIG. 5, i.e., from the developing sleeve 101 side toward the magnetic blade 202 side, between the developing sleeve 101 and the magnetic blade 202.
Accordingly, as shown in FIG. 6, tip ends of the magnetic toner T forms are shaped like ears of rice (state B) extending from the magnetic blade 202 to the developing sleeve 101. The imparting of charges to the magnetic toner T is done by the developing sleeve 101 and the toner t1 at the tip ends of the toner shaped like ears of rice formed extending from the magnetic blade 202 to the developing sleeve 101 coming into contact with each other to thereby impart charges to the tip ends.
Also, it has been found that the carrying of the toner between the developing sleeve 101 and the magnetic blade 202 is done as follows.
As described above, charges are imparted to the toner t1 at the tip ends of the ears of rice having contacted with the developing sleeve 101 and therefore, by a reflection force, a force acts toward the developing sleeve 101 and by the frictional force with the developing sleeve 101, a carrying force in the direction of rotation of the developing sleeve 101 is exerted.
Also, a certain degree of cohesive force acts between toners and therefore, a carrying force through the intermediary of the cohesive force is also created in the toner t2 which is in contact with the toner t1. A carrying force likewise through the intermediary of the cohesive force is also created in the toner t3 of the upper layer portion.
However, between the developing sleeve 101 and the magnetic blade 202, as described above, a magnetic force from the developing sleeve 101 side to the magnetic blade 202 side is also exerted on the toner. Accordingly, at a place where the carrying force exerted on the toner overcomes the above-mentioned magnetic force, i.e., on the cut line L of FIG. 6, the tip ends of the toner shaped like ears of rice are torn off, and the toner remaining on the developing sleeve 101 is carried in the direction of rotation of the developing sleeve 101.
Accordingly, in a system wherein the degree of cohesion of the magnetic toner is high or a system using a magnetic toner which is high in the frequency of necessary contact for obtaining a necessary triboelectric charge amount, there has been the problem that an insufficiently charged toner not in contact with the developing sleeve is carried to a developing area and a bad image resulting from bad charging is liable to occur.
In order to solve the foregoing problem, we have proposed a developing device which, as shown in FIG. 7 of the accompanying drawings, is provided with a developing sleeve 101 which is a first developer carrying member rotatably disposed in proximity to and opposed relationship with a photosensitive drum 108 which is a latent image bearing member bearing a latent image thereon and carrying a magnetic toner on the surface thereof, a developing sleeve 102xe2x80x2 which is a second developer carrying member having an axis parallel with the axis of the developing sleeve 101 and rotatably disposed in proximity to and opposed relationship with the developing sleeve 101 and carrying the magnetic toner on the surface thereof and regulating the magnetic toner on the developing sleeve 101 to a predetermined amount, and developer regulating means 105 for regulating the magnetic toner carried on the surface of the developing sleeve 102xe2x80x2 to a predetermined amount, and in which the magnetic toner from the developing sleeve 101 is imparted to the photosensitive drum 108 to thereby visualize the latent image as a toner image, and the developing sleeve 101 and the developing sleeve 102xe2x80x2 are rotated in the same direction and have fixedly disposed therein permanent magnets having a plurality of magnetic poles in the circumferential direction thereof.
Such a developing device is designed such that in a developer regulating portion which is a location at which the developing sleeve 101 and the developing sleeve 102xe2x80x2 are opposed to each other, at least two carrying forces acting in opposite directions are exerted to the magnetic toner and at least one of the above-mentioned carrying forces is a force which carries the magnetic toner toward a developing area and depends chiefly on the charged amount of the magnetic toner, and the carrying force working in the other direction than toward the developing area is a force depending on a magnetic force exerted on the magnetic toner, and the carrying force toward the developing area is not exerted to the uncharged magnetic toner. As the result, it has become possible to uniformly apply only the sufficiently charged magnetic toner to the surface of the first developer carrying member, and carry only the sufficiently charged magnetic toner to the developing area which is a location at which the photosensitive drum 108 and the developing sleeve 101 are opposed to each other.
However, in the developing device shown in FIG. 7, the amount of toner applied onto the developing sleeve 101 is ⅓ to xc2xd of that in the developing device shown in FIG. 4.
While in the developing device shown in FIG. 4, the value of (the rotational speed of the developing sleeve 101)/(the rotational speed of the photosensitive drum 108) (hereinafter referred to as the sleeve peripheral speed ratio) is often set to 1.2-1.5, in the developing device shown in FIG. 7, to obtain equal image density, the sleeve peripheral speed ratio must be made higher than in the developing device shown in FIG. 4.
In the developing device shown in FIG. 7, the developing efficiency is higher than in the developing device shown in FIG. 4 and therefore, the toner coat amount in about a half, but the sleeve peripheral speed ratio need not be double (2.5-3) that of the developing device shown in FIG. 4, and at a sleeve peripheral speed ratio of 2-2.5, image density equal to that in the developing device shown in FIG. 4 can be obtained.
In recent years, however, in accordance with the higher speed of the electrophotographic apparatus, the rotational speed of the photosensitive drum has also become higher and therefore, the rotational speed of the developing sleeve has necessarily become higher, and in the developing device shown in FIG. 7, the rotational speed of the developing sleeve must be made higher than in the developing device shown in FIG. 4.
In such a case, when use is made of a toner of a small particle diameter or a toner in which the amount of magnetic material is decreased, it has sometimes been difficult to stabilize the toner coating on the developing sleeve.
In order to solve the problem, there has been proposed a developing device which, as shown in FIG. 8 of the accompanying drawings, is provided with a developing sleeve which is a first developer carrying member rotatably disposed in proximity to and opposed relationship with a photosensitive drum 108 and carrying a magnetic toner thereon a developing sleeve 102 which is a second developer carrying member having an axis parallel with the axis of the developing sleeve 101 and rotatably disposed in proximity to and opposed relationship with the developing sleeve 101 and the photosensitive drum 108 and carrying the magnetic toner on the surface thereof and regulating the magnetic toner on the developing sleeve 101 to a predetermined amount, and developer regulating means 106 for regulating the magnetic toner carried on the surface of the developing sleeve 102 to a predetermined amount, and in which the magnetic toner from the developing sleeve 101 and the developing sleeve 102 is imparted to thereby visualize a latent image on the photosensitive drum 108 as a developer image, and the developing sleeve 101 and the developing sleeve 102 are rotated in the same direction and have fixedly disposed therein permanent magnets having a plurality of magnetic poles in the circumferential direction thereof.
Such a developing device is designed such that in a developer restraining portion which is a location at which the developing sleeve 101 and the developing sleeve 102 are opposed to each other, at least two carrying forces in opposite directions are exerted to the magnetic toner, and at least one of the above-mentioned carrying forces is a force which carries the magnetic toner toward a developing area and depends chiefly on the charged amount of the toner, and the carrying force working in the other direction than toward the developing area is a force depending on a magnetic force exerted on the magnetic toner from the developing sleeve 102, and the developing sleeve 101 and the developing sleeve 102 are disposed in proximity to the photosensitive drum 108.
In such a developing device, the carrying force wording toward the developing area which is a location at which the photosensitive drum 108 and the developing sleeve 101 are opposed to each other is a force depending chiefly on the charged amount of the toner, and the carrying force working in the other direction than toward the developing area is a force depending on the magnetic force exerted on the magnetic toner from the developing sleeve 102, whereby it is possible to carry only the sufficiently charged toner onto the developing sleeve 101.
As described above, any insufficiently charged toner is returned into the developing container 103 by the developing sleeve 102, and the toner coating the developing sleeve 101 is only the sufficiently charged toner.
The developing sleeve 102 is coated with the toner by a member having magnetism or an elastic member. This is a toner regulating method used in a developing device proposed in Japanese Patent Application Laid-Open No. 54-43036. In this method, the insufficiently charged toner also more or less coats the developing sleeve 102.
In the developing device shown in FIG. 8, both of the developing sleeves 101 and 102 are disposed in proximity to the photosensitive drum 108 are designed to effect development.
In the developing device shown in FIG. 8, the procedure of development is such that a latent image on the photosensitive drum 108 is first developed in the developing area by the magnetic toner from the developing sleeve 102. At this time, the toner image on the photosensitive drum 108 by development suffers from the occurrence of image inferiority such as fog or tailing due to the presence of the insufficiently charged toner, and is not sufficient in image density.
Next, the development of the latent image is effected by the magnetic toner from the developing sleeve 101. Here, the toner image formed on the photosensitive drum 108 is reciprocated in the developing area between the photosensitive drum 108 and the developing sleeve 101 by the AC of a developing bias applied onto the developing sleeve 101 and the photosensitive drum 108, and the insufficiently charged toner stripped off from the photosensitive drum 108 during the reciprocation is collected onto the developing sleeve 101 by the magnetic force from the magnetic poles in the developing sleeve 101.
As the result, development is finally effected by the sufficiently charged toner from the developing sleeve 101 and the sufficiently charged toner separated from the insufficiently charged toner from the photosensitive drum 108.
Thereby, it becomes possible to eliminate the image inferiority caused by the insufficiently charged toner and obtain sufficient image density without increasing the rotational speed of the sleeve.
As described above, in the developing device shown in FIG. 8, it has become possible to coat the developing sleeve 101 with only the sufficiently charged toner to thereby eliminate the image inferiority by the insufficiently charged toner caused on the photosensitive drum 108 by the developing sleeve 102.
Even in the developing device as shown in FIG. 8, however, there has been the possibility that depending on the magnetic force and the degree of cohesion or the like of the toner, the insufficiently charged toner moves from the developing sleeve 102 to the developing sleeve 101 and the developing sleeve 101 is coated with the insufficiently charged toner and becomes equal in the charged state to the developing sleeve 102, whereby an image defect by the insufficiently charged toner occurs.
It is an object of the present invention to provide a developing device in which after the amount of developer on a first developer carrying member is regulated, an insufficiently charged developer is prevented from moving from a second developer carrying member to the first developer carrying member.
It is another object of the present invention to provide a developing device which prevents the occurrence of a bad image by an insufficiently charged developer being present on a first developer carrying member.
It is another object of the present invention to provide a developing device in which magnetic poles in the interior of first and second developer carrying members are provided at such locations that it is difficult for a developer to move from a second developer carrying member to a first developer carrying member.
It is another object of the present invention to provide a developing device which can obtain sufficient image density without increasing the rotational speed of a developer carrying member.
Further objects and features of the present invention will become more apparent from the following detailed description when read with reference to the accompanying drawings.